Struct

In Swift 5.8, a struct is a value type: its data is stored directly in the variable's memory. This gives performance advantages, but also limits the type.

Structs are less often used in custom code than classes. For small units of data (like positions, sizes) structs are effective. They are copied when passed as arguments.

Struct versus `class`

Let us explore a key difference between structs and classes. When we create a struct, it is a value type. When we pass a struct to a func, it is copied.

Here We create a tiny class (TestClass) and a tiny struct (TestStruct). We pass them both to methods, as arguments.

Note The TestClass reference is copied, but not the data of the class. So we can change the class's inner data.

Note 2 A struct (like TestStruct) can be passed to a func, but its data cannot be modified in the func. It is a value, not a reference.

class TestClass {
    var code: Int = 0
}

struct TestStruct {
    var code: Int = 0
}

func increment(t: TestClass) {
    // The class instance is shared, so we can modify the memory.
    t.code += 1
}

func increment(t: TestStruct) {
    // The struct is copied, so cannot be modified in this func.
    // A new struct must be created.
}

// Create a class instance and modify it in a func.
var y = TestClass()
y.code = 1
increment(t: y)
print(y.code)

// Create a struct instance, which cannot be modified.
var x = TestStruct()
x.code = 1
increment(t: x)
print(x.code)2
1

Mutating

It is possible to modify the fields within a struct in a function call. But we must use the "mutating" keyword on the func signature.

Part 1 We have a mutating func within a struct called increaseBy, and it increments the value of the size field.

Part 2 When we create an Example struct instance, we can modify it by repeatedly calling the increaseBy function.

struct Example {
    var size = 0
    // Part 1: use mutating keyword on func inside struct.
    mutating func increaseBy(s: Int) {
        self.size += s
    }
}

// Part 2: call mutating func on struct to modify the existing struct.
var e = Example();
e.increaseBy(s: 5)
print("Size: \(e.size)")
e.increaseBy(s: 1)
print("Size: \(e.size)")Size: 5
Size: 6

Usage guidelines

When should we use structs instead of classes? For small units of data, where the fields will not change often after creation, structs are a good choice.

However For most custom types, classes are better. Structs have both performance advantages and negatives.

And When we pass a struct to a func, all its data is copied. This can be faster (if the struct is small) or slower (if it is big).

Structs have limitations. They are copied by value, so changes are disallowed or not reflected in the original variable. But structs can reduce allocations.

Struct

Struct versus class

Mutating

Usage guidelines

Struct versus `class`